Advanced Modeling Programs: Perplex-THERMOCALC Comparison

Dave Hirsch, Western Washington University and Julie Baldwin, University of Montana

Both Perplex and THERMOCALC have the ability to create a wide range of phase diagrams, pseudosections, and estimate mineral and rock properties.


THERMOCALC is in some ways a better pedagogical tool because it forces the student to really understand the Phase Rule, Schreinemaker's analysis, and how variance changes across field boundaries thus leading to a better understanding of how phase diagrams are constructed. Because of the method used (direct calculation of field boundaries), THERMOCALC produces smooth curves, with topologically correct Schreinemaker bundles.

THERMOCALC Disadvantages

Using THERMOCALC to create a diagram is quite laborious; each curve must be calculated by hand and the Schreinemaker's analysis must be done manually. The curves must be built up one by one, and manually combined. There is however, a program DRAWPD, for assembling the graphical output as a postscript file. Also, THERMOCALC has a substantial learning curve.

Perplex Advantages

Perplex's main strength is the very rapid ability to produce output. Complete pseudosections can be created in a few minutes with little understanding of the process. In addition, it has the ability to perform calculations with a wide range of thermodynamic databases and activity models. Activity models reside in a single file and can be chosen at run-time. It is updated very often, and graphical representations of rock or mineral properties are very easy to produce.

Perplex Disadvantages

Perplex's ease of use can also be a pedagogical drawback, in that students are apt to run the program without a deep consideration of their actions and choices. Perplex's methods of computation (calculation on a grid, division of solutions into discrete pseudocompounds) typically introduces small artifacts (akin to pixelization in an image) and may fail to identify features that are in close proximity to each other, leading to portions of diagrams that sometimes violate the Phase Rule. In the addition, the process of excluding phases and choosing solution models can be confusing for students and mistakes in this step can produce meaningless diagrams. Finally, Perplex is unable at this time to incorporate the 'equipartition constraint' used in some activity models by THERMOCALC.

Direct Comparison

These pseudosections were created by Mark Caddick (ETH) for direct comparison between THERMOCALC results and Perplex results. They use a pelitic bulk composition and the same activity models (expect for the equipartition difference). Note that due to the equipartition difference, the Chl+Ms+H2O field is larger in Perplex than THERMOCALC, and there are some differences in the high-T, low-P portion of the diagrams as well.

These images illustrate an important point: Perplex is an excellent tool for pre-visualizing pseudosections that can then be made using THERMOCALC. The lack of such a pre-visualization in the example below accounts for the high-T, low-P differences. In THERMOCALC, one must know in advance which minerals to consider. The THERMOCALC calculation did not consider sanidine, but the Perplex results show a reaction Ms+Qz = Sil + Kf + H2O at around 620°C and 2 kb (0.2 GPa). If sanidine had been considered, then the THERMOCALC results would include the extra fields shown in the Perplex pseudosection.

Pseudosection created by THERMOCALC for comparison with Perplex results.
Pseudosection created by Perplex_07 for comparison with THERMOCALC results.